Reduction of intercolor or bleeding in liquid ink printing

ABSTRACT

The present invention is directed to a method for processing color image data to reduce intercolor bleeding in an image printed on a receiving medium. The method begins with the receipt of a target pixel that includes a plurality of separation pixels corresponding to individual color planes, the color planes including a black plane and at least one non-black plane. The steps of the method further include determining if the target pixel is within an black border region near a black/color interface, and if so, modifying the target pixel using a substitution operation according to a black pixel modification pattern, the black pixel modification pattern identifying separation pixels in the black plane to replace with separation pixels from a non-black plane; and determining if the target pixel is within a color border region near a black/color interface, and if so, modifying the target pixel using a thinning operation according to a color pixel modification pattern, the color pixel modification pattern identifying a pattern of separation pixels in a non-black plane to remove from the composite image.

CROSS REFERENCE

Cross reference is made to the following related applications filedconcurrently herewith: “Adaptive Pixel Management Using Object TypeIdentification,” Torpey, et al., application Ser. No. 09/453,789,“Maintaining Black Edge Quality In Liquid Ink Printing,” Torpey et al.,application Ser. No. 09/453,788, “Identification Of Interfaces BetweenBlack and Color Regions,” Torpey et al., application Ser. No.09/454,152, “Reduction Of Intercolor Bleeding In Liquid Ink Printing,”Smith et al., application Ser. No. 09/453,791, and “Maintaining EdgeQuality In Liquid Ink Printing,” Curtis et al., application Ser. No.09/453,792.

BACKGROUND OF THE INVENTION

The present invention generally relates to liquid ink recording devicesusing two or more different color inks. More particularly, the presentinvention is directed to reducing intercolor bleeding that occurs at theinterface of areas printed with inks having different properties.

Liquid ink printers of the type often referred to as continuous streamor as drop-on-demand, such as piezoelectric, acoustic, phase changewax-based or thermal, employ at least one printhead from which dropletsof ink are directed towards a recording sheet. Within the printhead, theink is contained in a plurality of channels. Power pulses cause thedroplets of ink to be expelled as required from orifices or nozzles atthe end of the channels.

Liquid ink printers including ink jet printers deposit black and/orcolored liquid inks which tend to spread when the ink is deposited onpaper as a drop, spot, or dot. A problem of liquid ink printers is thatthe liquid inks used have a finite drying time, which tends to besomewhat longer than desired. Bleeding tends to occur when the drops areplaced next to each other in a consecutive order or in a cluster of dotswithin a short time. Bleeding, spreading, and feathering causes printquality degradation including color shift, reduction in edge sharpnessand solid area mottle which includes density variations in said areasdue to puddling of inks. Intercolor bleeding occurs when ink from onecolor area blends into or bleeds with ink from another color area.Intercolor bleeding is often most pronounced where an area of black ink(relatively slow drying) adjoins an area of color ink (relatively fastdrying); however, intercolor bleeding can occur at the interface betweenareas of any color inks having substantially different properties suchas dry time or permeability.

Various methods have been proposed to increase edge sharpness and toreduce intercolor bleeding. Some of the proposed methods includereplacing slow drying black ink with a process or composite black formedby combing fast drying color inks; under-printing a portion of the slowdrying black ink with a color ink, use a fast drying black ink, andusing both fast dry and slow dry black ink. While all of the proposedmethods reduce intercolor bleeding to some degree, they all have one ormore drawbacks that effect printer performance and/or image quality.

For example, using a fast dry ink in place of a slow drying black inkresults in a reduced quality of black reproduction as current fastdrying black inks have lower image quality than slow drying black inks.Additionally, fast drying black inks typically result in fuzzy edges inblack areas next to non-printed areas. The use fast drying black ink atan interface and slow drying black ink for interior regions caneliminate lower image quality associated with fast drying black inks,but increases the cost and complexity of printer design by requiring afifth ink in addition to the cyan, magenta, yellow and slow drying blackink. Similarly, replacing slow drying black ink with a process black(composite black) generated from fast drying color inks typicallyresults in a reduced quality of black reproduction resulting in a lowerimage quality than the use of slow drying black ink. Additionally, theuse of process black increases the amount of ink deposited on the printmedium, increases dry time and increase the time to print a document.Furthermore, the use of additional ink may not be suitable for printmedium such as transparencies and some types of paper which is not veryabsorbent. Under-printing a portion of the slow drying black ink with acolor ink can be used to reduce intercolor bleeding; however,under-printing increases the amount of ink on the print medium.Moreover, printing color under black often results in the thickening orblurring of edges particularly along edges between printed andnon-printed areas.

SUMMARY OF THE INVENTION

One aspect of the present invention is a process for processing colorimage data for printing on an inkjet printer to reduce intercolorbleeding in an image on a receiving medium. The process includes thesteps of: receiving color image data comprising a plurality of colorplanes that combine to form an array of composite pixels, the colorplanes including a black plane and a non-black plane, wherein each colorplane comprises an array of separation pixels; identifying an interfacebetween a black area and a color area; modifying the color image datacorresponding to an M-pixel wide color border in the color area using athinning operation according to a color pixel modification pattern, thecolor pixel modification pattern identifying a fraction of theseparation pixels in a non-black plane to remove from the color imagedata; and modifying the color image data within an N-pixel wide blackborder in the black area using a substitution operation according to ablack pixel modification pattern, the black pixel modification patternreplacing a fraction of the separation pixels in the black plane withseparation pixels in a non-black plane.

A second aspect of the present invention is a method of processing colorimage data for printing on an inkjet printer to reduce intercolorbleeding in an image on a receiving medium that includes receiving atarget pixel, the target pixel comprising a plurality of separationpixels each corresponding to an individual color plane, the color planesincluding a black plane and at least one non-black plane; determining ifthe target pixel is within an black border region near a black/colorinterface, and if so, modifying the target pixel using a substitutionoperation according to a black pixel modification pattern, the blackpixel modification pattern identifying separation pixels in the blackplane to replace with separation pixels from a non-black plane; anddetermining if the target pixel is within a color border region near ablack/color interface, and if so, modifying the target pixel using athinning operation according to a color pixel modification pattern, thecolor pixel modification pattern identifying a pattern of separationpixels in a non-black plane to remove from the composite image.

Another aspect of the present invention is a device for processing colorimage data reduce intercolor bleeding in an image on a receiving medium.The device includes a statistics collection filter connected to receivea target pixel and a set of surrounding pixels, the statisticscollection filter generating a statistics signal; a pixel identificationcircuit connected to receive the statistics signal, the identificationcircuit generating a pixel identification signal indicating whether thetarget pixel is within a border region; and a pixel modification circuitconnected to receive the pixel identification signal, the modificationcircuit modifying the target pixel according to a black bitmapmodification pattern when the target pixel is within a black border andmodifying the target pixel according to a color bitmap modificationpattern when the target pixel is within a color border.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of each drawing used to describethe present invention, and thus, are being presented for illustrativepurposes only and should not be limitative to the scope of the presentinvention, wherein:

FIG. 1 is a general representation of a suitable system-level embodimentfor one or more aspects of the present invention;

FIG. 2 is a diagram illustrating the steps of a process to reduceintercolor bleeding according to the concepts of the present invention;

FIG. 3 illustrates an example of a pixel substitution operation;

FIGS. 4 and 5 illustrate examples of a pixel thinning operations;

FIG. 6 shows an exemplary bitmap pattern for implementing a substitutionoperation;

FIGS. 7-10 illustrate the bitmap patterns for each of the individualcolor planes for the composite bitmap pattern of FIG. 6;

FIG. 11 shows an exemplary bitmap pattern for implementing a thinningoperation to eliminate all color pixels from every other compositepixel;

FIGS. 12-15 illustrate the bitmap patterns for each of the individualcolor planes for the composite bitmap pattern of FIG. 11;

FIG. 16 illustrates an arrangement for tiling bitmaps patterns overcolor image data;

FIG. 17 is a flow chart illustrating various steps in an embodiment of amethod for reducing intercolor bleeding according to the concepts of thepresent invention;

FIG. 18 illustrates a composite bitmap pattern that may be used tounder-print black pixels with color pixels;

FIG. 19 is a flowchart illustrating the steps of a process to maintainedge quality according to the concepts of the present invention;

FIG. 20 is a block diagram of a circuit for reducing intercolor bleedingin accordance with the present invention;

FIG. 21 is a flowchart showing a process for differentially processingobjects according to the concepts of the present invention;

FIG. 22 is a flowchart showing an embodiment of process fordifferentially processing objects in a document image;

FIG. 23 illustrates a document image comprising large and small text,graphics objects and a pictorial object; and

FIG. 24 is a partial schematic perspective view of an ink jet printersuitable for use with one or more aspects of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following will be a detailed description of the drawings illustratedin the present invention. In this description, as well as in thedrawings, like reference numbers represent like devices, circuits, orcircuits performing equivalent functions.

Turning now to FIG. 1, there is shown an embodiment of an exemplaryprinting system 10 that incorporates the features of the presentinvention. Printing system 10 includes image source 12 that may includescanner 14, computer 16, network 18 or any similar or equivalent imageinput terminal providing image data 20 which may be any combination ofASCII data, bitmapped image, geometric data, graphics primitives, pagedescription language, etc. Image data 20 is supplied to printer controlsystem 22 which processes the received image data 20 to produce printdata 24 that drives printer 26. Printer control system 22 may comprisewhat is commonly referred to in the art as a print driver. Those skilledin the art will recognize that control system 22 may be implemented inhardware and/or software and may reside within in image source 12,within printer 26, within a separate component or in any combinationthereof. In response to print data 24, which may comprise image dataand/or printer control signals (e.g., paper handling, carriage control,ink deposition), printer 26 generates an output image on a suitableprint medium. Beneficially, printer 26 may comprise an ink jet printer.

Turning now to FIG. 24, there is shown a partial schematic perspectiveview of an ink jet printer 200 suitable for use in the system of FIG. 1.Printer 200 includes an ink jet printhead cartridge 202 mounted oncarriage 204 supported by carriage rails 206. The printhead cartridge202 includes housing 208 containing ink for supply to printhead 210which selectively expels droplets of ink in response to control signalsreceived from controller 214 through a communication cable 212.Printhead 210 contains a plurality of ink conduits or channels (notshown) which carry ink from housing 208 to respective ink ejectors,which eject ink through orifices or nozzles (also not shown). Toeffectuate printing, controller 214 is coupled to one or more printheadcontrol circuits (not shown). The printhead control circuits receiveinformation from controller 214 via control signals received throughcommunication cable 212. In accordance with the content of the signalsreceived, the control circuits provide for selected ejection of inksfrom the nozzles of printhead 210.

When printing, carriage 204 reciprocates or scans back and forth alongcarriage rails 206 in the directions of arrow 216. As the printheadcartridge 202 reciprocates back and forth across a recording medium 218,such as a sheet of paper or transparency, droplets of ink are expelledfrom selected ones of the printhead nozzles towards the recordingmedium. During each pass of carriage 204, the recording medium 218 isheld in a stationary position. Upon the completion of one or morepasses, the recording medium is advanced in the direction of arrow 220by a feed mechanism under control of controller 214.

The present invention is directed towards aspects of printer controlsystem 22 depicted in FIG. 1. In particular, the present invention isdirected to a system and method for reducing intercolor bleeding thatoccurs at the interface between areas printed with inks havingsubstantially different properties such as dry time or permeability.Bleeding of colors may occur at the interface between color areas andsolid black areas and can lead to ragged edges and degraded printquality. As noted above, intercolor bleeding often occurs at theinterface between black and color areas as many ink jet printers combinea slow-drying black ink with fast-drying color inks. Thus, in describingthe present invention, reference will be made to intercolor bleedingoccurring at black/color interfaces. However, it is noted that theinvention is not limited to operating at black/color interfaces and maybe adapted to reduce intercolor bleeding occurring at the interfacebetween areas printed with color inks having substantially differentproperties. Furthermore, the present invention can be adapted to improveedge quality of black and/or color areas printed adjacent to anon-printed area.

The present invention is described as operating on color image datacomprising two or more color planes or separations that are combined toform a composite image. Each color plane comprises a raster imagedescribing the image for a color separation in terms of pixels arrangedin scanlines. For purposes of describing the present invention,reference will be made to image data comprising four color planes, Cyan,Magenta, Yellow and blacK (CMYK). Each composite pixel comprises fourassociated separation pixels, one for each of the CMYK color planes.Each separation pixel beneficially comprises a pixel value which may beconsidered as a binary signal indicating whether the correspondingseparation is on or off, i.e., whether the corresponding ink will bedeposited at that location or not. It will be appreciated that in aprinter which can deposit multiple ink drops of a single color at apixel location, a separation pixel may have multiple on states whereineach corresponds to depositing a different number of ink drops. Thoseskilled in the art will recognize that a different number of separationsas well as different combinations of colors may be used in forming acomposite image.

To reduce intercolor bleeding, the present invention carries out aprocess that operates to detect black/color interfaces where intercolorbleeding is likely to occur and to modify the pixels that are to beprinted near the borders of the interfaces The process comprises threegeneral steps: identifying an interface between a black area and a colorarea; modifying the pixel pattern in a black border region in the blackarea; and modifying the pixel pattern in a color border region in thecolor area. Referring to FIG. 2, there is shown a flowchart illustratingthis method for reducing intercolor bleeding in more detail.

Step 30 identifies an interface between a black area and a color area.In one embodiment, described in more detail below, step 30 collectsstatistics for pixels within a XxY window filter to identify aninterface and determine if a given pixel is within either border region.However, step 30 can use any number of known techniques including, butnot limited to, masking, look-up tables, edge detection filters, etc. toidentify a black/color interface. A discussion of edge detection filterscan be found in U.S. Pat. No 5,635,967.

Step 32 defines a width N of the black border region near theblack/color interface identified in step 30. The number of pixels Ncomprising the black border region should be large enough to reduceintercolor bleeding at the border and small enough to minimize theformation of additional printing artifacts that would likewise reduceimage quality. Similarly, step 34 defines the width M of the colorborder region near the interface. As with the selection of black borderregion, the width M of the color border region should be selected toreduce intercolor bleeding while minimizing the addition of otherprinting artifacts.

When defining the width of the border regions consideration may be givento such factors as the position of the border regions, the type of image(e.g., text, line art, graphics, pictorial, etc.), the width of eachborder, how the pixel pattern within a border will be modified, theprint medium used, ink composition, etc. Each of the border regionsbeneficially are positioned to abut the interface; however, it isunderstood that the border region need not abut the interface. The totalwidth of the border regions at an interface should be selected to reduceintercolor bleeding at an interface and minimize the formation ofadditional printing artifacts. Optimum values for border width can beidentified through calibration and image analysis studies. The width ofthe black border is preferably between 0 and 350 μm, and the width ofthe color border is preferably between 0 and 200 μm is used.Beneficially, for a 300 dpi ink jet the width of the N pixel blackborder is selected from 0 to 4 pixels, and the width M of the colorborder is defined to be from 0 to 2 pixels.

Steps 36 and 38 modify the pixel pattern within the N-pixel black borderand M pixel color border regions, respectively. A number of methodsexist to modify the pixels or pixel pattern within the border regions.One method of modifying the pixel pattern within a border regionreplaces selected pixels with a predetermined combination of separationpixels. The replacement operation effectively turns off the separationpixel this is being replaced and turns on the separation pixel(s)replacing it. The replacement of pixels is sometimes referred to as“substitution.” An example of a substitution operation is illustrated inFIG. 3. In FIG. 3, window 40 shows a 5×5 block of composite pixels alonga yellow/black interface. Window 42 shows the pixel block of window 40after a substitution operation wherein within a 2 pixel border (columns44 and 46) every other pixel in the black separation is turned off andreplaced with alternating cyan and magenta pixels in the compositeimage.

Another method of modifying a pixel pattern removes (turns off) selectedpixels in one or more separations from the composite image. This removalof pixels from separations is sometimes referred to as “thinning.” FIG.4 illustrates an example of a thinning operation wherein window 50 is a5×5 pixel block of a composite pixels along a black/color interface andwindow 52 shows the same image block after thinning. The thinningoperation removes (turns off) all color separation pixels from everyother pixel in column 54 and removes yellow separation pixels from everyother pixel in column 56.

A thinning operation can also be used to reduce the ink coverage in amultiple drop per pixel printer. Briefly, in a multi-drop per pixelprinter small ink drops are often used to produce good tone transitionsin graphical and pictorial images. However, the size of these drops arenot large enough to produce a solid area fill or saturated colors usingonly one drop per pixel. Thus, the printer typically requires greaterthan 100% coverage, that is, multiple drops per separation pixel toobtain solid area fill. In FIG. 5 window 60 illustrates a 5×5 pixel areaalong a black/color interface wherein the black region comprises 150%coverage (i.e., an average of three drops for every two pixels). Window62 shows the same image area as window 60 after a thinning operation toreduce the drop coverage to approximately 100%, Le., an average of onedrop per separation pixel. In window 62, column 64 illustrates athinning operation that reduces all two drop pixels to one drop pixels.Columns 66 and 68 illustrate a thinning method that removesapproximately half of the two drop pixels.

It should be appreciated that a pixel pattern may be modified using acombination of one or more substitution and/or thinning operations.Additionally, it should be appreciated that the thinning andsubstitution operations need not operate on pre-defined pattern ofpixels. For example, the pixel pattern modification may randomly selectone pixel of every three for thinning or substitution. Furthermore, whenoperating to modify the pixel pattern within a selected region, theoperation chosen to modify the pixel may vary based upon the positionwithin the region. Varying the pixel modification based upon pixelposition within the border allows for a transition within the borderregion to lessen any perceived dissimilarities between the border regionand the interior region. For example, the pixels closest to theinterface may be modified using a first substitution operation and thepixels farthest from the interface may be modified using a secondsubstitution operation.

Substitution and thinning operations to modify a pixel pattern can beimplemented by defining a pixel modification pattern for each differentsubstitution or thinning operation and adjusting pixels according to theappropriate pixel modification pattern. Each pixel modification patterncan, for example, be represented using an array of ON/OFF (1 or 0)values corresponding to separation pixels being turned on or off, i.e.,depositing ink or not, at specific locations in the cyan, magenta,yellow and/or black color planes. The substitution operation adjusts animage pixel by replacing the image pixel with the pixel value from thecorresponding pixel in the pixel modification pattern. A thinningoperation can adjust an image pixel by performing a logical AND of theimage pixel with the corresponding pixel value for an equivalentseparation in the pixel modification pattern.

For example, an exemplary pixel modification pattern 70 to implement asubstitution operation to replace every other pixel in the blackseparation with alternating cyan and magenta separation pixels is shownin FIG. 6. Pixel modification pattern 70 comprises four (4) scanlines 72with eight (8) pixels in each scanline. Modification pattern 70comprises composite pixels wherein the CMYK data is grouped together foreach pixel. For example, pixel 74 indicates that the correspondingseparation pixel is turned on in the magenta separation and turned offin the cyan, yellow, and black separations. FIGS. 7-10 illustrate 8×4modification patterns for each color plane, cyan, magenta, yellow andblack, respectively, for modification pattern 70 of FIG. 6. Similarly,an exemplary pixel modification pattern 80 for thinning a pixel patternto remove black from every pixel and colors in every other pixel isshown in FIG. 11. Modification pattern 80 comprises four (4) scanlines82 with eight (8) composite pixels of CMYK grouped data in eachscanline. FIGS. 12-15 illustrate 8×4 modification patterns for the cyan,magenta, yellow and black separations, respectively, for pixelmodification pattern 80 of FIG. 11.

Pre-defining each of the pixel modification patterns enables the pixelmodification patterns (arrays) to be tiled over the image data such thatan associated pixel location within each pixel modification pattern canbe identified for each pixel within the image data. FIG. 16 illustratesthe tiling of modification patterns over image data 90 comprising Jscanlines having I pixels in each scanline. Data 90 which may correspondto a composite image or a separation is shown with a plurality of XxYpixel modification patterns 92 tiled over the image. The correspondinglocation within a pixel modification pattern 92 can be identified forany pixel p(i, j) as a function of pixel position. Pixels within a blackor color border region near an interface can be modified according tothe pixel values in the appropriate pixel modification pattern.

Referring to FIG. 17, there is shown another embodiment of a process toreduce intercolor bleeding in accordance with the present invention. Theprocess of FIG. 17 is shown including an under-color printing(under-printing) operation. The addition of under-printing reduces drytime by adding additional fast-dry color ink to interior regions ofsolid slow-dry black ink in order to promote absorption of the black inkinto the paper. Furthermore, under-printing can reduce streakiness insolid black areas that may occur when slow-dry black ink does notreadily absorb and spread into the paper.

Briefly, the process illustrated in FIG. 17 operates on a colorcorrected, halftoned image. The process passes first window filterincluding a target pixel therein over a portion of the image, collectsstatistics for the pixels within the window filter, determines if thepixel is within an N-pixel black border region near a black/colorinterface based on the collected statistics and modifies the targetpixel according to pre-defined rules. A second window filter operatingon a second portion of the image including the target pixel may be usedto collect statistics for the pixels within the second window filter.The statistics collected from the second window are analyzed todetermine if the target pixel is within an M-pixel color border regionnear a black/color interface and, if so, the target pixel is modifiedaccording to a set of pre-defined rules. The reduction of intercolorbleeding operation modifies black pixels that lie within N pixels of aninterface with a color pixel and color pixels that lie within a M-pixelborder of a black pixel.

The under-printing operation modifies selected interior black pixels toturn on one or more color separation pixels, i.e., print color underblack at the selected pixel locations. The concept of “under-printing”or printing color under black is not exact, i.e., a printer may beconfigured and operated such that some pixels are printed with the colorink deposited under the black ink while other pixels are formed withblack ink deposited first and color ink deposited on top. The twooperations, intercolor bleeding and under-printing, are described asintegrated into a single process, although each can operateindependently.

More specifically, the process of FIG. 17 begins with the identificationa target pixel p(i,j) for processing in step 100. In step 102, theprocess determines if the composite pixel for the target pixel is ablack only pixel or a color only pixel. A black pixel is a compositepixel in which the separation pixel in the black plane is ON, i.e., atleast one drop of black ink will be deposited. A black only pixel is acomposite pixel in which the separation pixel in the black plane is ONand the separation pixels for each color plane are OFF. Similarly, in acolor only pixel the separation pixel in the black plane is OFF and atleast one separation pixel in a color plane is ON. If the target pixelis neither black only or color only, the process determines if there aremore pixels within the color image data to process (step 128). If so,the process loops back to step 100 for identification of a new targetpixel.

At step 104 the process branches based upon whether the target pixel isa color only pixel or a black only pixel. Each branch performs the samegeneral operations of collecting pixel statistics for the pixels withina window filter, analyzing the collected statistics to determine if thetarget pixel is within a border region near a black/color interface andprocessing the target pixel accordingly. Following the branch for blackonly target pixels, in step 106 the process identifies a black windowfilter comprising pixels surrounding the target pixel. Beneficially theprocess uses a square window filter centered on the target pixel. Thesize of the black window filter can be determined from the width of theborder region. For the N-pixel border region in the black area, the sizeof the black filter is beneficially set to have 2N+1 pixels on a side.

In step 108, the process determines if any pixels within the blackwindow filter have color under black (i.e. print both color and black atany pixel). If any pixel within the window has color under black, nofurther processing is performed for the target pixel. Step 108 is anoptional step in that the process need not be limited to black only,color only or non-printing pixels.

Step 110 collects statistics from the pixels within the black windowfilter to generate a black window statistics signal B(i,j) for thetarget pixel p(i,j). The statistics collected are used in subsequentprocessing (steps 112 and 114) to determine if the target pixel iswithin a N-pixel border region near a black/color interface and how, ifat all, the target pixel should be modified to reduce intercolorbleeding. It should be appreciated that the operation of step 108 can beincluded in the statistics collected at step 110. In one example,statistics signal B(i,j) identifies the number of black only and coloronly pixels within the black window filter in step 110. Based upon thenumber of black only and color only pixels within the black windowfilter, step 112 identifies if a black/color interface exists andidentifies a pixel type for the target pixel. In step 114 an outputpixel is generated from the target pixel by modifying the target pixelin accordance with a pre-defined pixel modification pattern based uponthe pixel type and whether an interface exists.

One method of identifying a pixel type determines if the number of blackpixels within the window is within a predetermined pixel range and, ifnot, whether the number of black pixels is over or under thepredetermined range. For example, the target pixel type may beidentified as an interior pixel if the number of black pixels is greaterthan the predetermined range, a border pixel if the number is within thepredetermined pixel range and an isolated pixel if the number is lowerthan the range. The method can be accomplished, for example, bycomparing the number of black only pixels to a series of thresholdvalues and setting the pixel type based upon the result. Additionally,if the number of color only pixels within the window is within aselected range, a black/color interface is presumed to exist. Theselected range for identifying a black/color interface may be defined bya predetermined lower threshold, by predetermined upper and lowerthresholds or by thresholds dynamically selected based upon the numberof black only pixels.

In general, if the target pixel is identified as a border pixel e.g.,within the N-pixel border region and a black/color interface is detectedwithin the black window filter, the target pixel is modified to reduceintercolor bleeding according to pixel modification pattern for blackborder pixels. If the target is within the N-pixel border region and aninterface is not detected, the pixel is left untouched. Optionally, ifthe target pixel is identified as an interior pixel (i.e., within asolid black area but outside the N-pixel border region), the pixel maybe modified according to an under-color print pixel modification patterndesigned improve image quality in large black areas.

An example of rules for identifying the pixel type of the target pixelbased upon the number of black only and color only pixels within thewindow filter described by the following C-like programming statement:

If (#_Black_Only > Border_Full) Pixel_Type = Interior

If ((Border_Empty ≦ #_Black_Only) and (#_Black_Only ≦ Border_Full))

Pixel_Type = Border

If (#_Black_Only < Border_Emptyl) Pixel_Type = Isolated

If (#_Color_Only ≧ ICB_Full) Bleed = Yes else BLEED = No

Wherein N is the number of pixels in the border region; the black windowfilter size is given by (2N+1)*(2N+1); #_Black_Only is the number ofblack only pixels within the window; #_Color_Only is the number of coloronly pixels within the window; Border_Full is the upper threshold foridentifying a border pixel and is given by (2N+1)*(2N+1) − N;Border_Empty is the lower threshold for identifying a border pixel andis given by (N+1)*(N+1); and ICB_Full is an interface threshold given byN.

General rules for modifying the target pixel based upon the pixel typeand existence of an interface can be described by the following C-likeprogramming statement:

If (Pixel_Type = Interior) generate output pixel by replacing targetpixel

according to an interior black pixel modification pattern

Else If ((Pixel_Type = Border) and (Bleed = Yes)) generate output pixelby

replacing target pixel according to a black border modification pattern

Else If ((Pixel_Type = Border) and (Bleed = No)) output target pixel

Else If ((Pixel_Type = Isolated) and (Bleed = Yes)) output black onlytarget

pixel

Else output target pixel

Turning to the color branch from step 104, the process identifies acolor window filter surrounding the target pixel at step 116. As withthe black pixel branch, the process beneficially uses a square windowfilter centered on the target pixel wherein the size of the color windowfilter is based upon width of the border region within the color areathat is modified; however, filters having other sizes and shapes mayalso be used. For an M-pixel border region the size of a square windowfilter is beneficially set to have 2M+1 pixels on a side.

Step 118 determines if any pixels within the color window filter havecolor under black. If any pixel within the window has color under black,no further processing is performed for the target pixel and the processis returned to step 100 for identification of a new target pixel ifthere are more pixels to be processed (step 128). Step 120 collectsstatistics for the pixels within the color window filter to generate acolor statistics signal C(i,j) for the target pixel p(i,j).

Based upon the statistics collected, step 122 determines if the targetpixel is within an M-pixel border region (e.g., a border pixel) near ablack/color interface. In general, the target pixel is presumed to bewithin an M-pixel border region near a black/color interface if thenumber of color only pixels within the window filter is within apredetermined range (greater than a first threshold and less then asecond threshold) and the number of black only pixels exceeds aninterface threshold.

An output pixel is generated from the target pixel in step 124. Ingeneral, if the target pixel is a border pixel and an interface exists,the target pixel is modified according to a color border pixel pattern.In all other cases, the target pixel is untouched and provided as theoutput pixel. General rules for determining if the target pixel is aborder pixel based upon the number of black only and color only pixelswithin the window filter can be described by the following C-likeprogramming statement:

If ((Border_Empty ≦ #_Color_Only) and (#_Color_Only ≦ Border_Full))

Pixel_Type = Border

If (#_Black_Only ≧ ICB_Full) Bleed = YES else BLEED = NO

If ((Pixel_Type = Border) and (Bleed = Yes)) generate output pixel by

thinning target pixel using a color border pixel modification pattern

Else If ((Pixel_Type = Border) and (Bleed = NO)) output target pixel

else output target pixel

Wherein M is the number of pixels in the border region; the color windowfilter size is given by (2M+1)*(2M+1); #_Black_Only is the number ofblack only pixels within the window; #_Color_Only is the number of coloronly pixels within the window; Border_Full = (2M+1)*(2M+1)−M;Border_Empty = (M+1)*(M+1); and ICB_Full = M.

After completing either branch, the process determines if there are morepixels within the color image data to process (step 128). If so, theprocess loops back to step 100 for identification of a new target pixel.The process ends when no further pixels need processing.

As discussed above, pre-defined pixel modification patterns comprisingan array of ON/OFF (1 or 0) values corresponding to pixels values beingturned on or off at specific locations in the color planes can be usedto implement thinning and substitution operations. The process of FIG.17 identifies three pre-defined pixel modification patterns: a colorborder pixel modification pattern, a black border pixel modificationpattern and an interior black pixel modification pattern. The specificpre-defined pixel modification patterns used by the process of FIG. 17to modify the target pixel may depend, in part, upon the type ofprinter, print mode (e.g., fast, slow, draft, normal, etc.), type ofimage (e.g., text, line art, graphic, pictorial), etc. For example, apixel printer may employ different pixel modification patterns forcontrolling intercolor bleeding for a draft mode than those used for anormal mode.

For a single drop per pixel printer, a black border pixel pattern tomodify pixels in black border regions near black/color interfaces thathas been found to provide good results replaces a fraction of the blackpixels with magenta pixels and a fraction of the black pixels with cyanpixels. Beneficially, the black border pixel modification patternreplaces approximately 50% of the black pixels with alternating cyan andmagenta pixels in a regular pattern. Preferably, the black border pixelmodification pattern replaces approximately 25% of the black pixels withcyan pixels and approximately 25% of the black pixels with magentapixels. An example of such a pixel modification pattern was discussedabove and shown in FIGS. 6-10. Another black border pixel modificationpattern which has been found to reduce intercolor bleeding replaces afraction of the black pixels with a regular pattern of cyan, magenta andyellow pixels. A pre-defined interior black pixel modification patternoperates to turn on (add) additional color separation pixels underselected black only pixels. Beneficially, the interior pixelmodification pattern under-prints a regular pattern of cyan, magenta andyellow pixels. An example of such a pixel modification pattern forprinting under approximately 25% of the black pixels is shown in FIG.18.

A color border pixel modification pattern which has been found to reduceintercolor bleeding thins the color border to remove a fraction of thecolor pixels within the pixel border. The thinning operation performedon the color border can remove any fraction of the color pixels up to,and including, all of the color pixels. Removing all of the color pixelcreates a non-printed area at the interface and is sometimes referred toas etching. Beneficially, the color border pixel modification patternremoves between 25 percent and 75 percent of the color pixels in thepixel border in a regular pattern. An example of such a pixelmodification pattern as illustrated in FIGS. 11-15 operates to removeapproximately half of the color pixels in a checkerboard pattern in a 2pixel border. It is understood that other combinations of thinning maybe used as well.

For a multiple drop per pixel printer, pixel modification patternsdesigned to reduce the maximum ink coverage in the border areas havebeen found to reduce intercolor bleeding. For border pixels in both theblack border and color border regions reducing drop coverage byreplacing some fraction of the multiple drop pixels with one drop pixelscan be used to reduce intercolor bleeding. Optionally, a pixelmodification pattern that thins a fraction of the multiple drop pixelswithin the border region can be used. Beneficially, the pixel patternsreduce ink drop coverage to approximately 100%, i.e., an average of onedrop per separation pixel in the black border and color border regions.However, reducing the ink drop coverage to between 100% and 150% incolor borders also provides good results. It should be appreciated thatother values for border widths and ink coverage may be appropriate basedon printer resolution, the inks used, the print medium used, dotscheduling algorithms, etc. The pixel modification patterns necessary toreduce ink drop coverage, either by replacing some multiple drop pixelswith one drop pixels or removing some of the multiple drop pixels,depend upon the maximum number of drops per pixel, the maximum dropcoverage that can be produced and the pattern in which the multipledrops are distributed.

It should be appreciated that the above described processes, asillustrated in FIG. 8 or 17, can be modified to identify pixels that arewithin a border region adjacent a non-printed area. The modified processoperates to identify pixels within a border region near aprinted/non-printed interface and modify the pixel pattern within theborder region of the printed area to sharpen or maintain edge quality.FIG. 19 illustrates a flowchart of a method to maintain edge quality.The modified process identifies interfaces between a printed area and anon-printed area, e.g., between black and non-printed areas and/orbetween color and non-printed areas (step 130); defines an N-pixelborder within a black area near a black/non-printed interface (step 132)and an M-pixel border within a color area near a color/non-printedinterface (step 134), and modifies the pixel pattern within the N-pixelblack border and M pixel color border regions (steps 136 and 138,respectively).

Identifying an interface between a printed area and a non-printed area(step 130) is similar to identifying an interface between a black areaand a color area and can be accomplished using many of the sametechniques including statistics collection, masking, look-up tables,edge detection filters, windowing, etc. Furthermore, the process of FIG.17 can be varied to identify printed/non-printed interfaces. Theadjusted process can include two window filters. Statistics collectedfrom a first window filter are analyzed to determine if a target pixelis within a N-pixel black border region near a black/non-printedinterface. Statistics from the second window filter are analyzed todetermine if a target pixel is within a M-pixel color border region neara color/non-printed interface. Optionally, the process can use a singlefilter for identifying both black/non-printed and color/non-printedinterfaces.

Briefly, a black window filter generates a black window statisticssignal B(i,j) that is analyzed to determine if the target pixel p(i,j)is within a N-pixel border region near a black/non-printed interface. Inone example, the target pixel is presumed to be within an M-pixel borderregion near an interface if the number of black pixels within the windowfilter is within a predetermined range and the number of color onlypixels is less than an interface threshold.

An example of rules for identifying if the target pixel is within aborder region near an interface based upon the number pixels within thewindow filter and modifying the target pixel are described by thefollowing C-like programming statement:

If (#_Black_Pixels > Border_Full) Pixel_Type = Black Interior

If ((Border_Empty ≦ #_Black_Pixels) and (#_Black_Pixels ≦ Border_Full))

Pixel_Type = Black Border

If (#_Black_Pixels < Border_Emptyl) Pixel_Type=Black Isolated

If (#_Color_Pixels < INT_Threshold) Modify = Yes else Modify = No

If ((Pixel_Type = Border) and (Modify = Yes)) generate output pixel by

replacing target pixel according to a black edge modification pattern

Else output target pixel

Wherein N is the number of pixels in the border region; the black windowfilter size is given by (2N+1)*(2N+1); #_Black_Pixels is the number ofblack pixels within the window; #_Color_Pixels is the number of coloronly pixels within the window; Border_Full is the upper threshold foridentifying a border pixel and is given by (2N+1)*(2N+1)−N; Border_Emptyis the lower threshold for identifying a border pixel and is given by(N+1)*(N+1); and INT_Threshold is an interface threshold given by N.

Similarly, the color window filter generates a color statistics signalC(i,j) that is analyzed to determine if the target pixel P(i,j) iswithin a M-pixel border region near a color/non-printed interface. Ingeneral, the target pixel is presumed to be within an M-pixel borderregion near an interface if the number of color only pixels within thewindow filter is within a predetermined range and the number of blackpixels is less than an interface threshold. An example of rules foridentifying if the target pixel is within a border region near aninterface based upon the number pixels within the window filter andmodifying the target pixel are described by the following C-likeprogramming statement:

If (#_Color_Pixels > Border_Full) Pixel_Type = Color Interior

If ((Border_Empty ≦ #_Color_Pixels) and (#_Color_Pixels ≦ Border_Full))

Pixel_Type = Color Border

If (#_Color_Pixels < Border_Emptyl) Pixel_Type = Color Isolated

If (#_Black_Pixels < INT_Threshold) Modify = Yes else Modify = No

If ((Pixel_Type = Border) and (Modify = Yes)) generate output pixel by

replacing target pixel according to a color edge modification pattern

Else output target pixel

Wherein M is the number of pixels in the border region; the color windowfilter size is given by (2M+1)*(2M+1); #_Color_Pixels is the number ofcolor only pixels within the window; #_Black_Pixels is the number ofblack pixels within the window; Border_Full is the upper threshold foridentifying a border pixel and is given by (2M+1)*(2M+1)−M; Border_Emptyis the lower threshold for identifying a border pixel and is given by(M+1)*(M+1); and INT_Threshold is an interface threshold given by M.

Optionally, when collecting statistics the pixels within a window filterand identifying a pixel type based upon the collected statistics, theprocess may compare the number of printing and non-printing pixels. Forexample, if the target pixel is a black only printed pixel, thecollected statistics may identify the number of black only pixels andthe number of non-printed pixels within the window filter. Rules foridentifying if the target pixel is a border pixel based upon the numberof black only and non-printed pixels within the window filter can bedescribed by the following C-like programming statement:

If ((Border_Empty ≦#_Black_Pixels) and (#_Black_Pixels ≦ Border Full))

Pixel_Type = Border

If (#_Non_Prinited ≧ INT_Threshold) Modify=YES else Modify = NO

The width of the border regions near the printed/non-printed interface(steps 132 and 134) should be selected to maintain/improve edge qualitywithout introducing printing artifacts that would reduce image quality.When defining the width of the border regions consideration may be givento such factors as the type of image (e.g., text, line art, graphics,pictorial, etc.), how the pixel pattern will be modified, the printmedium, ink composition, etc. Optimum values for border width can beidentified through calibration and image analysis studies. The width ofthe black border is preferably between 0 and 520 μm, and the width ofthe color border is preferably between 0 and 200 μm. Beneficially, for a300 dpi ink jet the width N of a border region within a black area isdefined to be between 0 to 6 pixels, and the width M of the color borderis selected to be from 0 to 2 pixels.

At steps 136 and 138 the pixel pattern within the N-pixel and M-pixelborders are modified to maintain edge quality. As with the modificationof border regions near a black/color interface, the pixel modificationpattern used may vary based on factors such as the printer type (singledrop, multi-drop), print mode (e.g., fast, slow, draft, normal, etc.),type of image (e.g., text, line art, graphic, pictorial), etc. For asingle drop per pixel printer, removing all color pixels under blackwithin a black border region near black/non-printed interface has beenfound to improve/maintain edge quality of the black area. Brieflyreviewing, under-printing a slow-dry black ink with a fast-dry color inkprovide improved dry time and a reduction in perceived banding orstreaks in large black areas. However, when fast dry ink is printedunder a black area, the edges of the black area adjoining a non-printedarea are poorly defined and often appear ragged. Thus, to maintain edgequality of a black area next to a non-printed area, a pixel modificationpattern may operate to remove (thin) all the color under black forpixels within an M-pixel border region within a black area bordering thenon-printed area. In addition to removing color under black in a borderregion, increasing the coverage by printing multiple drops at a pixelcan be used to improve edge quality.

In a multi-drop per pixel printer, edge quality of both black and colorareas printed adjacent to non-printed areas can be improved if allpixels within a border region have the same number of drops per pixel.For example, a multi-drop printer may limit the maximum coverage insolid black areas to 150% to prevent streakiness or pooling as well asto reduce dry-time. However, with 150% coverage some pixel locationsreceive two drops while other receive only one drop. The one-drop andtwo-drop pixels spread differently into the print medium, producing aragged edge. Thus, a pixel modification pattern that replaces all orsubstantially all one drop pixels with two drop pixels (increasingcoverage to 200%) provides good results. Alternatively, replacing alltwo drop pixels with one drop pixels (decreasing coverage to 100%) canalso be used to improve edge quality.

Referring now to FIG. 20, there is shown a block diagram of a circuit,according to one embodiment of the invention, for determining whether apixel is within a border region near an interface and modifying thepixel to reduce intercolor bleeding or improve edge quality. Asillustrated in FIG. 20, image data is fed to a first statisticscollection filter 140 which collects statistics for a target pixelp(i,j) and the neighboring pixels within a black border window andproduces a black border statistics signal B(i,j). The image data is alsofed to a second statistics collection filter 142 which collectsstatistics for the pixels within a color border window and produces acolor border statistics signal C(i,j). The statistics collected byfilter 140 and filter 142 may identify the number of black only pixels,number of color only pixels, number of non-printed pixels, color underblack pixels, etc.

The black border and color border statistics signals are fed to a pixelidentification circuit 144. Pixel identification circuit 144 analyzesthe statistics signals produce a pixel identification signal l(i,j) forthe target pixel p(i,j). The pixel identification signal l(i,j)identifies a pixel type for the target pixel. For example, circuit 144may analyze the target pixel to determine if the target pixel is a coloror black pixel, if the target pixel is within an N-pixel or M-pixelborder region and produce an identification signal that indicates thetarget pixel is one of a black interior pixel, a black border pixel, acolor border pixel, an isolated pixel, etc. The pixel identificationsignal l(i,j) is fed to a pixel modification circuit 146 operating onthe target pixel p(i,j). Depending upon the pixel identification signal,the modification circuit either allows the target pixel p(i,j) to passthrough unprocessed or modifies the pixel data associated with thetarget pixel according to an appropriate pixel modification pattern.

One skilled in the art would understand that the filters and circuitsdescribed above can embody or be implemented using a general or specialpurpose computer, a programmed microprocessor or microcontroller andperipheral integrated circuit elements, an ASIC or other integratedcircuit, a digital signal processor, a hardwired electronic or logiccircuit such as a discrete element circuit, a programmable logic devicesuch as a PLD, PLA, FPGA or PAL, or the like. Furthermore, specificalgorithms may be accomplished using software in combination withspecific hardware. In general, any device capable of implementing afinite state machine that is in turn capable of implementing a processdescribed above can be used to realize the associated filter or circuit.

The N-pixel and M-pixel border widths, the border threshold values andthe interface threshold values discussed above and utilized indetermining the pixel type and whether to modify the pixel as well asthe various pixel modification patterns can be either preset as defaultvalues or generated during a calibration process. Furthermore, severalsets of parameter values for the border widths and threshold values andseveral different pixel modification patterns can be stored andautomatically or manually selected for specified print modes or imagetypes. For example, faster print modes with fewer number of passes mightrequire intercolor bleed control processing to reduce dry time so thatit is comparable to the print speed and reduce the higher intercolorbleeding usually associated with faster printing. On the other hand,lower speed printing using more passes may not require such processing.

As indicated above, parameters such as pixel modification patterns andborder widths used for intercolor bleed control, under-printing or edgequality processing may be set (varied) based upon print mode (e.g.,draft, normal, high speed, low speed, etc.) and/or the image type (e.g.,text, line art, graphics pictorial, etc.). That is, the processes forreducing intercolor bleeding, under-printing and maintaining edgequality can be most effectively employed if they are applied selectivelyto different objects within a document based upon the image type andprint mode. For example, a process may be designed that provides asubstantial reduction of intercolor bleeding when applied to graphicsobjects but produces printing artifacts when applied to pictorialobjects.

To address this situation, processes for reducing intercolor bleeding(i.e., adjusting borders at black/color interfaces), under-printingand/or maintaining edge quality (i.e., adjusting borders atprinted/non-printed interfaces) can be applied on an object orientedbasis. That is, objects in a document can be classified as one ofseveral different types or classes such as text, line art, graphics,grayscale or pictorial. Each of the different classes can be assignedpreferred intercolor bleed control, under-printing and edge qualityprocesses. Moreover, analysis of the print quality obtained byapplication of various processes to different object types on differentprint mediums may be used to identify/describe various object classes.For example, consider black text on a non-printed (white) background.Small text generally has a small amount of ink and dries relativelyquickly. As the text becomes larger, however, a greater amount of ink isused and the drying time increases. Thus, text below a certain pointsize may not benefit (and may be harmed) from processing such asunder-printing to reduce dry-time. On the other hand, text above athreshold point size may benefit from under-printing processing toreduce dry-time to acceptable levels thereby improving print quality.

A similar analysis may be performed for black text on a color (printed)background. In general, intercolor bleeding increases when there is agreater amount of black ink available to bleed into colors. Thus,intercolor bleeding control and under-printing processes can beeffective in reducing bleeding and improving print quality for largerpoint size text. On the other hand, the use of intercolor bleedingcontrol and under-printing processing on small text printed on a colorbackground can produce visual defects such as lowered optical density,colored appearance, etc. Moreover, due to lower ink content black textbelow a threshold point size will generate a minimal amount of bleeding.Therefore, it may be preferable to eliminate the intercolor bleedingcontrol and under-printing processing for such text. Moreover, similarreasoning can be applied to lines, table borders, line art, etc. printedon non-printed (white) backgrounds and on printed (color) backgrounds.That is, thin lines (below a threshold width or point size) may notrequire intercolor bleeding control and under-printing processing andmay, in fact, produce a better visual appearance without suchprocessing. Likewise, broader lines would generally benefit fromintercolor bleeding control and under-printing processing by improvingdry-time and reducing intercolor bleeding.

The above analysis of print quality may be used to identify and supportseveral combinations of object classes. In one embodiment, each objectin a document may be classified into one of three classes: (a) blacktext below a selected font or point size threshold on an non-printed(white) background; (b) graphics objects including line art, black textlarger than a second font or point size threshold on a printed (color)background and black text greater than a selected threshold on annon-printed background and (c) pictorial objects. The processesdescribed above for reducing intercolor bleeding, under-printing andmaintaining edge quality may be applied to objects of class (b) withobjects of classes (a) and (c) receiving different pixel managementprocessing. It should be appreciated that different font and point sizethreshold parameters may apply for different classes as well as within aclass. For example, a first size threshold may be chosen for black texton a non-printed (white) background and a different size threshold mayapply for black text on a printed (color) background.

Alternatively, the above analysis of print quality may lead to thefollowing three classes: (a) text and line art smaller than a selectedpoint size threshold; (b) graphics objects including line art and texthaving a point size greater than a selected threshold and (c) pictorialobjects. Additionally, objects may be separated into “conventional”classes of text objects, graphics objects and pictorial objects whereinintercolor bleed control, under-printing and/or edge quality processingmay be applied to each text and graphics object based on the point orfont size and background type. Once again, it is understood thatdifferent size thresholds may be chosen for an object type based onwhether the object is on a printed or non-printed background as well asthe object type (e.g., text, line art, graphics). Furthermore, thoseskilled in the art will recognize that other object types may also bedefined.

To selectively apply intercolor bleed control, under-printing and/oredge quality processing on an object oriented basis, one embodiment ofthe present invention carries out a process as illustrated in FIG. 21.The process of FIG. 21 operates to identify objects within a documentimage, classify the objects into one of several predefined image types.Each of the different object types can be processed using a preferredset of processing techniques. More specifically, document image datawhich may include ASCII data, bitmapped image data (including colorcorrected, halftoned data), geometric data, graphics primitives, pagedescription language, etc. as well as any combination thereof isreceived in step 150.

At step 152, the document image data is analyzed to identify andclassify objects within the document image that are to be rendereddifferentially so as to result in an output image that is more desirablethan the unaltered input image. Toward this end, step 152 identifiesthree types of objects in the image: text, graphics and pictorial. It isunderstood that hybrids of these three basic classes, or entirelydifferent or additional/more specific classes, may be used depending onthe particular desired application. Objects within an image may beidentified and classified from an analysis of the image data. The formatof the image data received may be used to identify objects. That is,text objects may be bitmapped data or ASCII text characters, whilepictorial objects may be in the form of a multibit per pixel rasterimage. Graphics objects may be described as graphics primitives orgeometric data.

Similarly, for image data in a page description language, analysis ofthe image data can be used to identify graphics objects on the page andtheir attributes, such as size, border color, fill color, linethickness, and the like. The analysis can also provide information onhow and where text is used on the page, as well as the text attributes,such as text size, color, spacing and whether the text is next to, or ontop of colored regions. Moreover, the identification and classificationof objects within an image can be accomplished using any number of wellknow segmentation classification functions including, but not limitedto, auto-correlation, frequency analysis, pattern or template matching,peak/valley detection, histograms, etc. Furthermore, techniques forclassifying image data, such as those taught by Fan et al. in U.S. Pat.No 5,850,474 and by Revankar et al. in U.S. Pat. No 5,767,978, can beused to identify objects within the document image.

In the second stage, the objects classified as text objects, graphicsobjects or pictorial objects are processed according to the selected ordefault processing techniques most appropriate for processing a text,graphics or picture image region. For example, text objects can beprinted according to a first set of pixel management (e.g., intercolorbleed control, under-printing and edge quality processing) processingparameters, graphics objects according to a second set of pixelmanagement processing parameters, and pictorial objects according to athird set of pixel management processing parameters.

At step 154, text objects are directed to text processing techniques asillustrated with steps 160-168. If the text is on a printed (color)background (step 160), and is greater than a predetermined font or pointsize threshold (step 162), then intercolor bleeding control,under-printing and edge quality processing is performed (step 166).Similarly, if the text object has a non-printed (white) background (step160), and is greater than a predetermined point size threshold (step164), a first pixel management process which may include intercolorbleeding control, under-printing and edge quality processing optimizedfor large text objects is performed (step 166). On the other hand, fortext objects less than the predetermined size thresholds at steps 162and 164, an alternative pixel management process optimized for smalltext is performed (step 168). The alternative pixel management processof step 168 may include any combination of intercolor bleed control,under-printing processing and edge quality processing such as edgequality processing without intercolor bleed control or under-printingprocessing. Likewise, the pixel management processing of step 168 maynot include any intercolor bleed control, under-printing processing oredge quality processing.

The processing techniques applied to graphics objects (step 156) aresimilar to those applied to text objects. That is, in addition to imageprocessing functions (e.g., color matching or correction, halftoning,etc.), graphics objects that are greater than a predefined graphics sizethreshold receive a first set of graphics pixel management processingthat may include intercolor bleeding control, under-printing and edgequality processing while the graphics objects that are less than thegraphics size threshold receive a second set of graphics pixelmanagement processing. Finally, in addition to image processingfunctions (e.g., color matching or correction, halftoning, etc.),pictorial objects receive a set of pictorial pixel management functionsthat may include intercolor bleeding control, under-printing and/or edgequality processing optimized for pictorial objects.

As discussed above, any manner of differentiating image objects thatwill result in an improved quality image can be used. Thus, in certainembodiments of the system and method of the present invention,identification of images as text, graphics or pictures may be eliminatedin favor of a system involving different object classes. Such a systemmay allow greater latitude in differentially processing images than onebased on “traditional” image classes like text, graphics and pictorialobjects. For example, in the process of FIG. 21 alternative classes(e.g., class one, class two and class three) may be substituted for textobjects, graphics objects and pictorial objects, respectively. Whereinclass one may be defined to include black text and line art on annon-printed (white) background smaller than a selected font or pointsize. Class two may be defined to include graphics as well as text andline art on a color background larger than a selected point size andtext and line art on an non-printed background having a font or pointsize greater than a selected threshold. Class three would maintainpictorial objects. As previously described, classes one and three maynot need intercolor bleeding control and under-printing processingapplied thereto, while class two can benefit from receiving intercolorbleed control, under-printing and edge quality processing.

Referring to FIG. 22, there is shown a flowchart illustrating anotherembodiment of the present invention for differentially processingobjects within a document image. The process of FIG. 22 operates toidentify regions within the document image that include at least one ofa graphics object, line art or text on a printed or background largerthan a first threshold selected font or point size or line art or texton a non-printed background having a font or point size greater than asecond threshold.

The process of FIG. 22 begins with the receipt of a document image whichmay include any combination of text, graphics and/or pictorial objects(step 170). It is understood that the objects may or may not have beenpreviously identified and labeled as text, graphics or pictorials. Thedocument image comprises a one or more document regions each of whichcan be defined in terms of number of pixels, number scanlines, bytes ofimage data, blocks of image data, etc. The regions comprising thedocument image need not be equal in size and can be of any size up toand including the entire document. Moreover, the regions can bepredefined or identified dynamically as the document image data isreceived. For example, in a Microsoft® Windows® environment an image mayprinted using a technique that divides the image into printer bands andprocesses each band. With such a technique, each band processed whichcan be identified by the driver by a starting location, width and heightcan be considered as an image region.

At step 172, statistics for each image region are collected. At aminimum, the statistics collected will be used to identify image regionsthat include at least a portion of one of a graphics object, line art ortext on a color background larger than the first font or point size andline art or text on an non-printed background greater than the secondfont or point size threshold. The statistics collected may includesegmentation data, data describing the type of geometric objects on thepage and their attributes, such as size, border color, fill color, linethickness, and the like. Other statistics may include information on howtext is used on the page, as well as the text attributes, such as textsize, color and spacing and whether the text is next to, or on top ofprinted (color) areas. Moreover, step 172 may identify within a pagedescription language instructions for generating a graphics object suchas a graphical data interface call in a Windows® environment. Additionalstatistics collected in step 172 may include identifying black areasthat are adjacent to color areas, which objects are black only or coloronly objects, and which objects have a mixture of black and color.

At step 174, the process identifies whether each image region within thedocument image which may be classified as including a graphics object,line art or text on a color background larger than the first font orpoint size and line art or text on an non-printed background greaterthan the second font or point size threshold. Additionally, step 174 mayidentify those regions that include other types of objects as well(i.e., regions including text below a size threshold on a non-printedbackground or including pictorials). However, for intercolor bleedingcontrol, under-printing and edge quality processing step 174 need notspecifically identify any small text or pictorial objects or regionsincluding them.

At step 176, image regions identified in step 174 as including agraphics object, line art or text on a color background larger than thefirst font or point size and/or line art or text on an non-printedbackground greater than the second font or point size threshold areprocessed with a first pixel management process that includes intercolorbleeding control, under-printing and/or edge quality processingoptimized for such object types. Regions that do not include at leastone of a graphics object, line art or text on a color background largerthan the first font or point size and/or line art or text on annon-printed background greater than the second font or point sizethreshold are processed with a second pixel management process. Thesecond pixel management process may comprise any combination ofintercolor bleed control, under-printing processing and edge qualityprocessing. Alternatively, the second pixel management processes maysimply include operations to cause a printer to generate an outputdocument without performing intercolor bleed control, under-printingprocessing or edge quality processing.

The following description in conjunction with FIG. 23 provides oneexample of the process illustrated in FIG. 22 for differentiallyprocessing objects within a document image. FIG. 23 illustrates a sampledocument image 180 comprising area 182 of large text, a graphics object184 such as a bar chart, a pictorial object 186 and several areas 188 ofsmall text. Document 180 is further divided into several image regions190-196. In processing document image 180, the image data correspondingto image region 190 is received and statistics are collected the regionare collected. Based upon the collected statistics, the processdetermines that region 190 includes a large text and classifies region190 as including a graphics object, line art or text on a printedbackground larger than the first font or point size and line art or texton an non-printed background greater than the second font or point sizethreshold. Based upon this classification, the image data correspondingto region 190 is processed according to a first pixel management processthat includes at least one of intercolor bleed control, under-printingprocessing and edge quality processing. For example, the image datawithin region 190 may be processed according to the method describedabove with reference to FIG. 17.

The process similarly collects statistics for the image datacorresponding to regions 192, 194 and 196. From the collectedstatistics, the process classifies regions 192 and 194 as including agraphics object, line art or text on a printed background and/or lineart or text on an non-printed background. Based upon thisclassification, the image data within these regions, including the smalltext and pictorial image data are processed according to a first pixelmanagement process. Region 196, on the other hand, would not be soclassified, and thus, would be processed according to a second pixelmanagement process which may perform any combination, including none, ofintercolor bleed control, under-printing processing or edge qualityprocessing.

It should be appreciated that the above process of FIG. 22 is notlimited to the binary classification described above. For example, imageregions may be classified into one of the following three classes: class1—regions including a graphics object or large text/line art; class2—regions including both a pictorial object and one of a graphics objector large text/line art; and class 3—regions not including a graphicsobjects, large text/line art.

While the present invention has been described with reference to variousembodiments disclosed herein, it is not to be confined to the detailsset forth above, but it is intended to cover such modifications orchanges as made within the scope of the attached claims.

What is claimed is:
 1. A method of processing color image data forprinting on an inkjet printer to reduce intercolor bleeding in an imageon a receiving medium, comprising: receiving color image data comprisinga plurality of color planes that combine to form an array of compositepixels, the color planes including a black plane and a non-black plane,wherein each color plane comprises an array of separation pixels;identifying an interface between a black area and a color area;modifying the color image data corresponding to an M-pixel wide colorborder in the color area using a thinning operation according to a colorpixel modification pattern, the color pixel modification patternidentifying a fraction of the separation pixels in a non-black plane toremove from the color image data; and modifying the color image datawithin an N-pixel wide black border in the black area using asubstitution operation according to a black pixel modification pattern,the black pixel modification pattern replacing a fraction of theseparation pixels in the black plane with separation pixels in anon-black plane.
 2. The method of claim 1, wherein the M-pixel widecolor border abuts the interface and M is selected such that the widthof the color border is from 0 to 200 μm.
 3. The method of claim 1,wherein the M-pixel wide color border abuts the interface and M isselected such that the M-pixel wide color border is less than threepixels wide.
 4. The method of claim 1, wherein the N-pixel wide blackborder abuts the interface and N is selected such that the N-pixel wideblack border is less than five pixels wide.
 5. The method of claim 1,wherein the N-pixel wide black border abuts the interface and N isselected such that the width of the black border is from 0 to 350 μm. 6.The method of claim 1, wherein the color pixel modification patternthins approximately 50 percent of the separation pixels from allnon-black planes.
 7. The method of claim 6, wherein the black pixelmodification pattern replaces a first fraction of the black separationpixels with separation pixels in a first color plane and a secondfraction of the black separation pixels with separation pixels in asecond color plane.
 8. The method of claim 1, wherein the black pixelmodification pattern operates to replace a first fraction of theseparation pixels in the black plane with separation pixels in a firstcolor plane and a second fraction of the separation pixels in the blackplane with separation pixels in a second color plane.
 9. The method ofclaim 8, wherein the black pixel modification pattern further replaces athird fraction of the black separation pixels with separation pixels ina third color plane.
 10. The method of claim 1, wherein approximatelyhalf of the black separation pixels being replaced are replaced withseparation pixels in a first non-black plane and approximately half ofthe black separation pixels being replaced are replaced with separationpixels in a second non-black plane.
 11. The method of claim 10, whereinthe color of the non-black separation pixel replacing the blackseparation pixel is based upon the position of the composite pixelwithin the black border.
 12. The method of claim 1, further comprising:identifying an interior black region within the black area; andmodifying the composite image within the interior black region accordingto an interior pixel modification pattern, the interior pixelmodification pattern adding non-black separation pixels to the compositeimage.
 13. A method of processing color image data for printing on aninkjet printer to reduce intercolor bleeding in an image on a receivingmedium, comprising: receiving a target pixel, the target pixelcomprising a plurality of separation pixels each corresponding to anindividual color plane, the color planes including a black plane and atleast one non-black plane; determining if the target pixel is within anblack border region near a black/color interface, and if so, modifyingthe target pixel using a substitution operation according to a blackpixel modification pattern, the black pixel modification patternidentifying separation pixels in the black plane to replace withseparation pixels from a non-black plane; and determining if the targetpixel is within a color border region near a black/color interface, andif so, modifying the target pixel using a thinning operation accordingto a color pixel modification pattern, the color pixel modificationpattern identifying a pattern of separation pixels in a non-black planeto remove from the composite image.
 14. The method of claim 13, whereinthe black pixel modification pattern identifies a pattern of blackseparation pixels to replace and wherein a first fraction of the blackseparation pixels in the pattern are replaced with separation pixels ina first color plane and a second fraction of the black separation pixelsin the pattern are replaced with separation pixels in a second colorplane.
 15. The method of claim 14, wherein the step of determining ifthe target pixel is within a black border region comprises: determiningif a first condition is met, the first condition being that the numberof black pixels within a black window filter is within a black borderpixel range; determining if a second condition is met, the secondcondition being that the number of color only pixels within the blackwindow filter is greater than an interface threshold; and identifyingthe target pixel as being within a black border region near an interfacewhen the first and second conditions are met.
 16. The method of claim13, wherein the color pixel modification pattern thins all separationpixels in non-black planes from approximately 50 percent of the pixellocations within color pixel modification pattern.
 17. The method ofclaim 16, wherein the step of determining if the target pixel is withina color border region comprises: determining if a first condition ismet, the first condition being that the number of color only pixelswithin a color window filter is within a border pixel range; determiningif a second condition is met, the second condition being that the numberof black pixels within the color window filter is greater than aninterface threshold; and identifying the target pixel as being within acolor border region near an interface when the first and secondconditions are met.
 18. A device for processing color image data toreduce intercolor bleeding in an image on a receiving medium,comprising: a statistics collection filter connected to receive a targetpixel and a set of surrounding pixels, the statistics collection filtergenerating a statistics signal; a pixel identification circuit connectedto receive the statistics signal, the identification circuit generatinga pixel identification signal indicating whether the target pixel iswithin a border region; and a pixel modification circuit connected toreceive the pixel identification signal, the modification circuitmodifying the target pixel according to a black bitmap modificationpattern when the target pixel is within a black border and modifying thetarget pixel according to a color bitmap modification pattern when thetarget pixel is within a color border.
 19. The device of claim 18wherein the statistics collection filter comprises: a black statisticscollection filter operating on a first set of neighboring pixels togenerate a black statistics signal; and a color statistics collectionfilter operating on a second set of neighboring pixels, the colorstatistics collection filter generating a color statistics signal. 20.The device of claim 19, wherein: the black statistics signal identifiesthe number of black only pixels and the number of color only pixelswithin the set of surrounding pixels; and the color statistics signalidentifies the number of black only pixels and the number of color onlypixels within the set of neighboring pixels.
 21. The device of claim 19,wherein: the black bitmap modification pattern is designed to replace afraction of the black pixels with regular pattern of non-black pixels;and the color bitmap modification pattern is designed to thin a portionof the color pixels within a border region.